1. Field of the Invention
This invention relates to a transmitter for communication devices as presented in the preamble of claim 1 and to a mobile station as presented in the preamble of claim 9.
2. Description of the Prior Art
The transmitters of communication devices have a high frequency power amplifier, in which the signal to be transmitted is amplified. The output of the high frequency power amplifier is connected to the adapter circuit of the antenna, in which circuit the impedance of the antenna is adapted to the output impedance of the high frequency power amplifier. The purpose of the adaption is, among other things, to prevent the formation of reflection waves from the antenna towards the high frequency power amplifier. However, high frequency power amplifiers are sensitive to load variations. Load variations cause distortion in the signal to be amplified, among other things. It is also possible that the high frequency power amplifier is damaged in difficult load conditions. In portable communication devices, in particular, the load variations of high frequency power amplifiers are due to the interaction between the antenna and the operation environment and changes in operating conditions. Metal objects in the vicinity of the antenna, for example, can remarkably change the antenna impedance of the portable communication device. This, in turn, has an effect on the operation point of the last stage of the high frequency power amplifier, whereby the transistor is exposed to large voltage and current variations. In time, these voltage and current variations can impair the performance of the output stage transistor of the high frequency power amplifier, and possibly also shorten its lifetime.
There are prior art solutions, in which the power signal formed by the high frequency power amplifier is measured by means of a directional coupler and a detector diode. For example, FIG. 1 shows a prior art coupling, in which the directional coupler DIR1 samples the power fed to the output. The samples are detected by a detector diode D1. A method like this, based on a directional coupler, operates well when the load impedance Z is constant. However, the method provides incorrect information in situations in which the load impedance varies, as usually happens when portable communication devices are used. In order to indicate this, the operation of the coupling in FIG. 1 has been simulated. The simulation results can be seen in FIGS. 2a-2e. In this simulation, a bipolar transistor biased into the class AB was used as the power transistor T1 of the output stage, and a harmonic trap was used to form the harmonics. Samples of the output power were taken by a directional coupler DIR1, and the samples were detected with a detector diode D1. The detector diode D1 was biased to the linear region of operation, whereby the output power is proportional to the square of the voltage V.sub.meas formed by the detector diode D1.
Load variations are common in portable communication devices, such as mobile stations, because the interaction between the environment and the antenna cause load variations in the high frequency power amplifier. Table 1 shows various impedance values used in the simulation. In the first simulation, the value of the load impedance Z was such that it resulted in an optimum resistive load for the simulated amplifier. Different values of the load impedance Z were used in other simulations, resulting in an incorrect adaptation. The values used correspond to a reflection loss of -6 dB to a 6 ohmn load. Simulation results with different values of load impedance are shown in FIGS. 2a-2e. The power measurement was calibrated to produce the correct power reading with an output power of two watts. FIGS. 2a-2e show both the square of the voltage V.sub.meas formed by the detector diode D1 and the output power P.sub.out of the amplifier in different load situations.
TABLE 1 Simulation no. Value of the load impedance Z 1 6 .OMEGA. 2 2 .OMEGA. 3 18 .OMEGA. 4 3.6 + j4.8 .OMEGA. 5 3.6 - j4.8 .OMEGA.
It can be seen from FIG. 2a that the power measurement gives an accurate result in optimum load conditions. It can further be seen from FIGS. 2b-2e that when the load of the amplifier varies, the square of the detected voltage is no more the same as the power conveyed to the load, whereby the measurement does not give the correct idea of the load situation of the amplifier.
Still another disadvantage of using a directional coupler is the fact that a directional coupler causes power loss in the signal to be transmitted. In practical applications, the directional coupler is typically implemented by means of conductor traces incorporated directly on the printed circuit board (PCB), whereby the power loss of the directional coupler is typically approx. 0.5 dB. In addition, a directional coupler formed directly on the circuit board takes an unnecessarily large amount of space.